INDUSTRIAL TRAINING PROJECT

REPORT

ON

UTTARAKHAND POWER

CORPORATION LTD.

33/11 KV SUBSTATION

UTTARKAASHI

Submitted To: Submitted by:

Vinay Vashisht

ECE 7th sem

11-ECE-1616

1

ACKNOWLEDGEMENT

With profound respect and gratitude, I take the opportunity to convey my

great thanks to complete the training here, since training has very important

role in exposing real life situation in an industry.

I am extremely grateful to all the technical staff of UTTARKAASHI

substation UPCL, UTTARKAASHI for their co-operation and guidance that

helped me a lot during the course of training. I have learnt a lot working

under them and I will always be indebted of them for this value addition in

me.

I would also like to thank the training in charge of Rohtak Institute Of Tech.

& Management , Rohtak and all the faculty member of Electronics And

Communications Engineering department for their effort of constant co-

operation which have been significant factor in the accomplishment of my

industrial training.

2

CONTENTS

S.R. NO.

TOPIC PAGE NO

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

UPCL-AN OVERVIEW TRAINING AT UPCL(UTTARKAASHI) TRANSFORMER COMPONENTS OF TRANSFORMER TYPES OF TRANSFORMER SUBSTATION EARTHING MATERIAL BUS BAR INSULATOR MISCELLANEOUS EQUIPMENT PROTECTION OF SUBSTATION CIRCUIT BREAKER PROTECTION AGAINST LIGHTENING

3 4 6 10 12 14 17 20 21 22 23 25 28

3

An Overview

Uttarakhand, the 27th State of India was created on 9th November 2000 as the 10th Himalayan State of the country blessed with the natural and mineral resources in abundance and poised to be a 20000 MW HYDRO POWER HUB of India in the

future.

Uttarakhand Power Corporation Ltd (UPCL), formerly Uttaranchal Power Corporation Ltd was incorporated under the Companies Act, 1956 on February 12, 2001

consequent upon the formation of the State of Uttaranchal. UPCL has been entrusted to cater to the Transmission & Distribution Sectors inherited after the de merger from

UPPCL since 1st April 2001. The Electricity Act. 2003 mandated the separation of Transmission functions under Power Sector Reforms. On 1st June 2004, the Power

Transmission Corporation Limited (PTCUL) was formed to maintain & operate 132 KV & above Transmission Lines & substations in the State. Today UPCL,

the State Power Distribution Utility of the Government of Uttaranchal (GOU) caters to the Sub –Transmission & Distribution Secondary Substations & Distribution Lines 66 KV & below in the State .UPCL - the Frontline State Power Distribution Utility &

service provider of QUALITY & RELIABLE POWER SUPPLY to over 1.59 million consumers of electricity spread over the 13 Districts of Uttarakhand.

These electrical consumers are categorized depending on their domestic, commercial,

agricultural and industrial loads. UPCL is also the first electrical utility in India to initiate women empowerment by employing local women through Self Help Groups,

as franchisees, for meter reading, bill distribution and revenue collection.

UPCL looks forward to a committed participation from a Team of professionals always striving for performance excellence with new innovative technologies to strengthen the

Power Distribution Infrastructure of the STATE in Seamless Integration with Generation & Transmission Utilities for the Socio – economic development.. A

comprehensive POWER EVACUATION PLAN is underway with construction of new 33/11 KV Substations in the State.

4

Training at UPCL (UTTARKAASHI) I was appointed to do training from this organization from 20th June to 19rd July 2013. In this duration I was assigned under the supervision of Mr. Shakti Prasad for

understanding distribution section, UPCL.

Definition of sub-station: “The assembly of apparatus used to change some characteristics of electric supply is called sub-station”.

Introduction: The present day electrical power system is a.c. electric power is generated, transmitted, and distributed in the form of Alternating current. The electric power is produce at the power station, which are located at favorable places, generally

quite away from the consumers. It is delivered to the consumer through a large network of transmission and distribution. At many place in the line of power system, it may be

desirable and necessary to change some characteristic of electric supply. This is accomplished by suitable apparatus called sub-station for example, generation voltage

(11KV or 6.6KV) at the power station is stepped up to high voltage (Say 220KV to 132KV) for transmission of electric Power. Similarly near the consumer’s localities,

the voltage may have to be stepped down to utilization level. Suitable apparatus called sub-station again accomplishes this job.

About the substation: The substation in UTTARKAASHI, Uttarakhand is one of the

important power grids in the state of Uttarakhand. Cause it supplies the Barkot Industrial area & locality. The most important of any substation is the grounding

(Earthing System) of the instruments, transformers etc. used in the substation for the safety of the operation personnel as well as for proper system operation and

performance of the protective devices. An earthen system comprising of an earthing mat buried at a suitable depth below ground and supplemented with ground rods at

suitable points is provided in the substations. These ground the extra high voltage to the ground. As it is dangerous to us to go near the instrument without proper earth. If

the instruments are not ground properly, they may give a huge shock to anyone who would stay near it and also it is dangerous for the costly Instrument as they may be

damaged by this high voltage

Site Selection of 33 KV Substation: 33KV Sub-Station forms an important link between Transmission network and Distribution network. It has a vital Influence of reliability of service. Apart from ensuring efficient transmission and Distribution of

5

power, the sub-station interruptions in power Supply. Sub-Station is constructed as near as possible to the load center. The voltage level of power transmission is decided

on the quantum of power to be transmitted to the load center. Transmission is decided on the quantum of power to be transmitted to the load center.

Selection of site: Main points to be considered while selecting the site for Grid Sub-

Station are as follows:

i) The site chosen should be as near to the load center as possible. ii) It should be easily approachable by road or rail for transportation of equipment.

iii) Land should be fairly leveled to minimize development cost. iv) Source of water should be as near to the site as possible. This is because water is

required for various construction activities (especially civil works), earthing and for drinking purposes etc.

v) The sub-station site should be as near to the town / city but should be clear of public places, aerodromes, and Military / police installations.

vi) The land should have sufficient ground area to accommodate substation

equipments, buildings, staff quarters, space for storage of material, such as store yards and store sheds etc. with roads and space for future expansion.

vii) Set back distances from various roads such as National Highways, State Highways should be observed as per the regulations in force.

viii) While selecting the land for the Substation preference to be given to the Govt. land over private land.

ix) The land should not have water logging problem. x) Far away from obstructions, to permit easy and safe approach /termination of

high voltage overhead transmission lines. configuration should be such that it enables easy maintenance of equipment and minimum

6

Transformer

Transformer is a static machine, which transforms the potential of alternating current at

same frequency. It means the transformer transforms the low voltage into high voltage & high voltage to low voltage at same frequency. It works on the principle of static

induction principle. The transformer is an electromagnetic conversion device in which electrical energy received by primary winding is first converted into magnetic energy

which is reconverted back into a useful electrical energy in other circuits (secondary winding, tertiary winding, etc.). Thus, the primary and secondary windings are not connected electrically, but coupled magnetically. A transformer is termed as either a

step-up or step-down transformer depending upon whether the secondary voltage is higher or lower than the primary voltage, respectively. Transformers can be used to

either step-up or step-down voltage depending upon the need and application; hence their windings are referred as high-voltage/low-voltage or high-tension/low-tension

windings in place of primary/secondary windings.

Transformers mainly are of three types:

1. Step up transformer 2. Isolators

3. Step down transformer

Figure: Transformer

7

Major Transformer in Power Plant

Generator Transformer:

In generator transformer the generator is connected to this transformer by the means of isolated bus-ducts. This transformer is used to step up the generated voltage to the grid

voltage. It is generally provided with OFB cooling and oil immersed circuit taps on the high voltage side are also present. The transformer has an elaborate cooling system

consisting of a number of oil pumps and cooling fan apart from various accessories described later.

GT Specification:

Rating(MVA) 125

Type of cooling OFB

Temp. rise of oil 45°C

kV (No Load) HV 233Kv

LV 10.5kV

Phase HV 3

LV 3

8

Unit Auxiliary Transformer: It draws input from main bus duct connecting the generating transformer. Total KVA

capacity unit auxiliary transformer required can be determined by using 0.85 p.f.& 0.9 efficiency for total auxiliary motor load. It is usually safe and desired to provide 20% excess capacity then calculated to provide miscellaneous auxiliaries.

UAT Specification:

1. UAT Rating (MVA) 12.5

2. Volts at NO Load (KVA) HV- 10.5

LV- 6.6

3. Line Current HV- 687 A

LV- 1047 A

4. Phase HV- 3

LV- 3

5. Type of Cooling ON

6. Frequency 50 Hz

7. Vector Group Symbol Delta- delta

8. Insulation Level HV- 75 kV (peak)

LV- 60 kV (peak)

9. Temperature Rise of Oil 450 C

10. Temperature Rise of Winding 550 C

9

Station Transformer:

This transformer is placed after generator transformer. It steps down 220 kV to 440 V

for plant electricity like lighting, drinking water purpose and for general purpose motor (not LT or HT motors which run by UAT). It works for all the time and in case of plant

shutdown, these transformers are fed by the national grid and also fulfill the purpose of UAT for the plant startup.

Different Components of a Transformer:

Magnetic circuit:

It consists of a high permeance core over which both primary and secondary coils are

wound. Electrical energy transfer between two circuits takes place through a transformer without the use of moving parts; the transformer therefore has higher

efficiency and low maintenance cost as compared to rotating electrical machines.

Winding:

The rectangular paper-covered copper conductor is the most commonly used conductor for the windings of medium and large power transformers. These conductors can be

individual strip conductors, bunched conductors or continuously transposed cable (CTC) conductors. In low voltage side of a distribution transformer, where much fewer

turns are involved, the use of copper or aluminum foils may find preference. To enhance the short circuit withstand capability, the work hardened copper is commonly

used instead of soft annealed copper, particularly for higher rating transformers. In the case of a generator transformer having high current rating, the CTC conductor is

mostly used which gives better space factor and reduced eddy current losses in windings.

Conservator Tank:

A small tank placed on the top of main tank. It is half filled with air and half filled with oil. It maintains the level of oil in transformer. If oil level falls air comes in conservator

through the breather to fill the vacuum created.

10

Breather: It performs the function of releasing and taking atmospheric air. Further it is filled with

silica gel to prevent the contamination of transformer oil in the conservator by the moisture present in the air entering the conservator.

Cooling Mechanism:

Low power transformers are generally air cooled. For large power transformers, air cooling is used. Oil performs the dual role of a coolant (heat exchanger) and an

insulating medium.

Explosive Vent: In case of severe fault in the transformer, the internal pressure may be build up to a

very high level, where it may result in an explosion of tank. Therefore this vent is provided to remove the excess pressure from transformer if any such situation arises.

Buchholz Relay:

This relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment. On a slow accumulation of gas, due perhaps to slight overload,

gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level down. A float switch in the relay is used to initiate an alarm signal.

If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to isolate the apparatus.

Mulsifire Mechanism: This is provided for protection in case of a fire break-out in the transformer. A pipe

filled with pressurized air at 2-3 kg/cm2 is connected to a glass bulb. This pressure stops a valve which operates the flow of water through nozzles provided over the entire

tank. In case of a fire, the glass bulb shatters due to the heat, releasing the pressurized air. This fall in pressure causes the mulsifire valve to open, releasing water sprays from

the nozzle, thereby quenching the fire.

11

SWITCH YARD Switch yard is a switching yard and is defined as the enclosed areas at the power

stations containing switching facilities and equipment for the purpose of connecting to the transmission network. It consists of Isolators, circuit breakers, Current

Transformers, Potential Transformers, Capacitance Voltage Transformers, Wave Trap and Lightning Arrestors. Switchyard forms an integral part of any power station i.e., thermal Power Utilities,

Gas turbines based power plants or Hydel power plants. Switchyard will exist at a generating station to coordinate the exchange of power the generators and the

transmission lines in the area.

Basic Structure of a switch yard

Current Transformer is used for measuring high current and Potential Transformer is used for measuring high voltage.

Capacitance Voltage Transformers are used for maintaing the constant voltage in case of voltage drop in transmission line.

Isolators is off-load switching device which disconnects the connection between busbars in off condition. It id on both the sides of switch yard.

In BTPS, SF6 circuit breakers are used disconnect the connection between busbars in both off-load and on-load condition.

Wave Trap is used for sending and receiving of wave through transmission lines. It is basically used for detecting any fault in transmission line.

Lightning Arrestors are used to supress the high voltage formed due to lightning to ground.

TYPES OF TRANSFORMER

1 Power transformer

12

2 Instrument transformer

3 Auto transformer

4 On the basis of working

5 On the basis of structure

Power Transformer:

1.Single phase transformer

2.Three phase transformer

Instrument Transformer

1.Current transformer

2. Potential transformer

Auto Transformer

1. Single phase transformer

2. Three phase transformer

ON THE BASIS OF WORKING

Step down: Converts high voltage into low voltage.

Step up: Converts low voltage into high voltage.

13

ON THE BASIS OF STRUCTURE

Figure: core type

Figure: Shell type

14

SPECIFICATION OF C.T.

Figure : Current transformer

1 Standard: IS-2785

2 Highest System Voltage: 145 KV

3 Frequency: 50Hz

4 C.T. Current: 25 KA/1Sec.

5 Rated primary current: 800 Ampere

SUBSTATIONS

The present day electrical power system is A.C. i.e. electrical power is generated, transmitted & distributed in the form of the alternating current. The electric power is

produced at power plant stations which are located at favorable places generally quite away from the consumers. It is delivered to the consumers through a large network of

transmission 7 distribution.

At many places in the power system, it may be desirable and necessary to change some characteristics e.g. voltage, ac to dc, frequency, power factor etc. of electric supply.

This accomplished by suitable apparatus called substation. For example; generation voltage (11 KV or 33 KV) at the power station is set up to high voltage (say 220 KV or

15

132 KV) for transmission of electric power. The assembly of apparatus (e.g. transformer etc.) used for this purpose in the substation. Similarly near the consumer’s

localities, the voltage may have to be step down to utilization level. This job is again accomplished by suitable apparatus called substation.

The assembly of apparatus to change some characteristic of electric power supply is called substation.

TYPES OF SUBSTATION

According to the service requirement:

Transformer substation Switch substation

Power factor correction substation Frequency change substation

Converting substation Industrial substation

According to the constructional features:

Indoor substation Outdoor substation

Underground substation Pole mounted substation

TRANSFORMER SUBSTATION

They are known as transformer substations as because transformer is the main

component employed to change the voltage level. depending upon the purposed served transformer substations may be classified into:

STEP UP SUBSTATION

The generation voltage is steeped up to high voltage to affect economy in transmission of electric power. These are generally located in the power houses and are of outdoor

type.

PRIMARY GRID SUBSTATION

Here, electric power is received by primary substation which reduces the voltage level to 66KV for secondary transmission. The primary grid substation is generally

16

SECONDARY SUBSTATIONS

At a secondary substation, the voltage is further steeped down to 11KV. The 11KV lines runs along the important road of the city. The secondary substations are also of

outdoor type.

DISTRIBUTION SUBSTATION

These substations are located near the consumer’s localities and step down to 400V, 3-phase, 4-wire for supplying to the consumers. The voltage between any two phases is

400V & between any phase and neutral it is 230V.

SUBSTATION CHARACTERISTICS:

Each circuit is protected by its own circuit breaker and hence plant outage does

not necessarily result in loss of supply.

A fault on the feeder or transformer circuit breaker causes loss of the transformer

and feeder circuit, one of which may be restored after isolating the faulty circuit

breaker.

A fault on the bus section circuit breaker causes complete shutdown of the

substation. All circuits may be restored after isolating the faulty circuit breaker.

Maintenance of a feeder or transformer circuit breaker involves loss of the

circuit.

Introduction of bypass isolators between bus bar and circuit isolator allows circuit breaker maintenance facilities without loss of that circuit.

STEPS IN DESIGNING SUBSTATION:

The First Step in designing a Substation is to design an Earthing and Bonding System.

Earthing and Bonding:

The function of an earthing and bonding system is to provide an earthing system

connection to which transformer neutrals or earthing impedances may be connected in order to pass the maximum fault current. The earthing system also ensures that no thermal or mechanical damage occurs on the equipment within the substation, thereby

resulting in safety to operation and maintenance personnel. The earthing system also

17

guarantees equipotent bonding such that there are no dangerous potential gradients developed in the substation.

In designing the substation, three voltage have to be considered these are:

Touch Voltage:

This is the difference in potential between the surface potential and the potential at

earthed equipment whilst a man is standing and touching the earthed structure.

Step Voltage:

This is the potential difference developed when a man bridges a distance of 1m with

his feet while not touching any other earthed equipment.

Mesh Voltage:

This is the maximum touch voltage that is developed in the mesh of the earthing

grid.To determine the effective substation earthing resistance, from which the earthing voltage is calculated.In practice, it is normal to take the highest fault level for

substation earth grid calculation purposes. Additionally, it is necessary to ensure a sufficient margin such that expansion of the system is catered for.To determine the

earth resistivity, probe tests are carried out on the site. These tests are best performed in dry weather such that conservative resistivity readings are obtained.

Earthing Materials

Conductors:

Bare copper conductor is usually used for the substation earthing grid. The copper bars themselves usually have a cross-sectional area of 95 square millimeters, and they are

laid at a shallow depth of 0.25-0.5m, in 3-7m squares. In addition to the buried potential earth grid, a separate above ground earthing ring is usually provided, to which

all metallic substation plant is bonded.

Connections:

18

Connections to the grid and other earthing joints should not be soldered because the heat generated during fault conditions could cause a soldered joint to fail. Joints are

usually bolted, and in this case, the face of the joints should be tinned.

Earthing Rods:

The earthing grid must be supplemented by earthing rods to assist in the dissipation of

earth fault currents and further reduce the overall substation earthing resistance. These rods are usually made of solid copper, or copper clad steel.

Switchyard FenceEarthing:

The switchyard fence earthing practices are possible and are used by different utilities.

These are:

Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then bonded to the grid at regular intervals.

Place the fence beyond the perimeter of the switchyard earthing grid and

bond the fence to its own earthing rod system. This earthing rod system is not coupled to the main substation earthing grid.

CONDUCTORS USED IN SUBSTATION DESIGN:

An ideal conductor should fulfills the following requirements:

Should be capable of carrying the specified load currents and short time

currents.

Should be able to withstand forces on it due to its situation. These forces

comprise self weight, and weight of other conductors and equipment, short

circuit forces and atmospheric forces such as wind and ice loading.

Should be corona free at rated voltage.

Should have the minimum number of joints.

Should need the minimum number of supporting insulators.

Should be economical.

19

The most suitable material for the conductor system is copper or aluminums. Steel may be used but has limitations of poor conductivity and high susceptibility to corrosion.

In an effort to make the conductor ideal, three different types have been utilized, and these include: Flat surfaced Conductors, Stranded Conductors, and Tubular Conductors

Overhead Line Terminations

Two methods are used to terminate overhead lines at a substation.

Tensioning conductors to substation structures or buildings

Tensioning conductors to ground winches.

The choice is influenced by the height of towers and the proximity to the substation. The following clearances should be observed:

VOLTAGE LEVEL MINIMUM GROUND CLEARANCE

less than 66kV 6.1m

66kV - 110kV 6.4m

110kV - 165kV 6.7m

greater than 165kV 7.0m

POWER LINE CARRIER COMMUNICATION

Introduction:

Reliable & fast communication is necessary for safe efficient & economical power supply. To reduce the power failure in extent & time, to maintain

the0020interconnected grid system in optimum working condition; to coordinate the operation of various generating unit communication network is indispensable for state

electricity board. In state electricity boards, the generating & distribution stations are generally located at

a far distance from cities. Where P & T communication provided through long overhead lines in neither reliable nor quick. As we have available very reliable

20

physical paths viz. the power lines, which interconnected, hence power line carrier communication is found to be most economical and reliable for electricity boards.

APPLICATIONS:

The PLCC can be used for the following facilities:

Telephony

Teleportation

Remote control or indication

Telemetry

Teleprinting

BUSBARS

When numbers of generators or feeders operating at the same voltage have to be

directly connected electrically, bus bar is used as the common electrical component. Bus bars are made up of copper rods operate at constant voltage. In large stations it is

important that break downs and maintenance should interfere as little as possible with continuity of supply to achieve this, duplicate bus bar system is used. Such a system

consists of two bus bars, a main bus bar and a spare bus bar with the help of bus coupler, which consist of the circuit breaker and isolator.

In substations, it is often desired to disconnect a part of the system for general

maintenance and repairs. An isolating switch or isolator accomplishes this. Isolator operates under no load condition. It does not have any specified current breaking

capacity or current making capacity. In some cases isolators are used to breaking charging currents or transmission lines. While opening a circuit, the circuit breaker is

opened first then isolator while closing a circuit the isolator is closed first, then circuit breakers. Isolators are necessary on supply side of circuit breakers, in order to ensure isolation of the circuit breaker from live parts for the purpose of maintenance. A

transfer isolator is used to transfer main supply from main bus to transfer bus by using bus coupler (combination of a circuit breaker with two isolators), if repairing or

maintenance of any section is required.

21

INSULATORS

The insulator serves two purposes. They support the conductors (bus bar) and confine the current to the conductors. The most common used material for the manufacture of

insulator is porcelain. There are several types of insulators (e.g. pin type, suspension type, post insulator etc.) and their use in substation will depend upon the service

requirement. For example, post insulator is used for bus bars. A post insulator consists of a porcelain body, cast iron cap and flanged cast iron base. The hole in the cap is

threaded so that bus bars can be directly bolted to the cap.

With the advantage of power system, the lines and other equipment operate at very high voltage and carry high current. The arrangements of switching along with switches cannot serve the desired function of switchgear in such high capacity circuits.

This necessitates employing a more dependable means of control such as is obtain by the use of the circuit breakers. A circuit breaker can make or break a circuit either

manually or automatically under all condition as no load, full load and short circuit condition.

A circuit breaker essentially consists of fixed and moving contacts. These contacts can

be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of breaker get energized and the moving contacts

are pulled apart by some mechanism, thus opening the circuit. When contacts of a circuit breaker are separated, an arc is struck; the current is thus

able to continue. The production of arcs are not only delays the current interruption, but is also generates the heat. Therefore, the main problem is to distinguish the arc

within the shortest possible time so that it may not reach a dangerous value. The general way of classification is on the basis of the medium used for arc extinction.

MISCELLANEOUS EQUIPMENT:

Capacitor Bank: The load on the power system is varying being high during

22

morning and evening which increases the magnetization current. This result in the decreased power factor. The low power factor is mainly due to the fact most

of the power loads are inductive and therefore take lagging currents. The low power factor is highly undesirable as it causes increases in current, resulting in

additional losses. So in order to ensure most favorable conditions for a supply system from engineering and economical stand point it is important to have

power factor as close to unity as possible. In order to improve the power factor come device taking leading power should be connected in parallel with the load.

One of the such device can be capacitor bank. The capacitor draws a leading current and partly or completely neutralize the lagging reactive component of

load current.

Figure: Capacitor bank

Capacitor bank accomplishes following operations:

Supply reactive power

Increases terminal voltage

Improve power factor

A fuse is a short piece of wire or thin strip which melts when excessive current through

it for sufficient time. It is inserted in series with the circuit under normal operating conditions; the fuse element is at a nature below its melting point. Therefore it carries

the normal load current overheating. It is worthwhile to note that a fuse performs both detection and interruption functions.

23

PROTECTION OF SUBSTATION

Transformer protection: Transformers are totally enclosed static devices and generally oil immersed. Therefore

chances of fault occurring on them are very easy rare, however the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected

from the system. This provides adequate automatic protection for transformers against possible faults.

Conservator and Breather:

When the oil expands or contacts by the change in the temperature, the oil level goes either up or down in main tank. A conservator is used to maintain the oil level up to

predetermined value in the transformer main tank by placing it above the level of the top of the tank.

Breather is connected to conservator tank for the purpose of extracting moisture as it spoils the insulating properties of the oil. During the contraction and expansion of oil

air is drawn in or out through breather silica gel crystals impregnated with cobalt chloride. Silica gel is checked regularly and dried and replaced when necessary.

Marshalling box:

It has two meter which indicate the temperature of the oil and winding of main tank. If

temperature of oil or winding exceeds than specified value, relay operates to sound an alarm. If there is further increase in temperature then relay completes the trip circuit to

open the circuit breaker controlling the transformer.

24

Transformer cooling:

When the transformer is in operation heat is generated due to iron losses the removal of heat is called cooling.

There are several types of cooling methods, they are as follows:

Air natural cooling:

In a dry type of self cooled transformers, the natural circulation of surrounding air is used for its cooling. This type of cooling is satisfactory for low voltage

small transformers.

Air blast cooling:

It is similar to that of dry type self cooled transformers with to addition that

continuous blast of filtered cool air is forced through the core and winding for better cooling. A fan produces the blast.

Oil natural cooling:

Medium and large rating have their winding and core immersed in oil, which act both as a cooling medium and an insulating medium. The heat produce in the

cores and winding is passed to the oil becomes lighter and rises to the top and place is taken by cool oil from the bottom of the cooling tank.

Oil blast cooling:

In this type of cooling, forced air is directed over cooling elements of

transformers immersed in oil.

Forced oil and forced air flow (OFB) cooling:

Oil is circulated from the top of the transformers tank to a cooling tank to a

cooling plant. Oil is then returned to the bottom of the tank.

Bus-bar :- When a no. of lines operating at the same voltage have to be directly connected electrically, bus-bar are used, it is made up of copper or aluminum bars

(generally of rectangular X-Section) and operate at constant voltage. The bus is a line in which the incoming feeders come into and get into the instruments

for further step up or step down. The first bus is used for putting the incoming feeders in LA single line. There may be double line in the bus so that if any fault occurs in the one, the other can still have the current and the supply will not stop. The two lines in

the bus are separated by a little distance by a Conductor having a connector between them. This is so that one can work at a time and the other works only if the first is

having any fault.

25

Circuit breaker :

A circuit breaker is an equipment, which can open or close a circuit under normal as well as fault condition. These circuit breaker breaks for a fault which can damage other

instrument in the station. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault condition. A

circuit breaker consists of fixed & moving contacts, which are touching each other under normal condition i.e. when breaker is closed. Whenever a fault occurs trip coil

gets energized, the moving contacts are pulled by some mechanism & therefore the circuit is opened or circuit breaks. When circuit breaks an arc is stack between

contacts, the production of arc not only interrupts the current but generates enormous amount of heat which may cause damage to the system or the breaker itself. Therefore

the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that the heat generated by it may not reach a dangerous value. The

medium used for arc extinction is usually Oil, Air, Sulfur Hexafluoride (SF6) or vacuum.

Circuit breakers can be classified on the basis of medium used for arc

extinction: A. Oil Circuit Breakers:-

These are the oldest type of circuit breakers & have the virtues of reliability, simplicity of construction & relative cheapness. These are mainly of two types:

a. Bulk Oil Circuit Breakers using large quantity of oil are also called the dead

tank type because the tank is held at earth potential. Such circuit breakers may further be classified as:-

i. Plain Break Oil Circuit Breakers are very simple in construction & widely

used in low voltage d.c & a.c circuits. For use on higher voltages, they become unduly large in size & need huge of transformer oil. In addition, such breakers are not suitable for high-speed interruption; therefore, these cannot be used in

auto-closing.

ii. Self-Generated Pressure Oil Circuit Breakers are of three types viz. Plain

explosion pot having limited breaking capacity, cross jet explosion pot suitable

26

for interrupting heavy current t high voltage (66kV) & self-compensated explosion pot suitable for operation both at heavy currents as well as low

currents. Plain explosion pot cannot be used either for very low currents because of increased arcing time or for very heavy currents because of risk of bursting of

pot due to high pressure.

iii. Impulse Type Oil circuit Breakers have the main advantage, over other conventional design, of reduced requirement of oil (roughly one-fourth). The

possibility of current chopping can also be avoided by using resistance switching.

b. Low oil or Minimum Oil Circuit Breakers are also called the live tank circuit

breakers because the oil tank is insulated from the ground. Such circuit breakers are now available for all type of voltages (3.6, 7.2, 12, 36, 72.5,145,245 & 420 kV) &

for the highest breaking capacities. The MOCB with rated voltage of 12 kV has a single interrupter per phase without extra support insulator.

B. Low Voltage Air Circuit Breakers:-

These breakers are designed for use on d.c circuits & low voltage a.c circuits for the protection of general lighting & motor circuits. These breakers are usually provided with an over current tripping mechanism which may be of instantaneous or time delay

type or combination of both. Trip devices may be set over a range from about 80 to 160 percent of rating. The breakers may also be provided with over tripping ranges &

arrangements such as low voltage trip, shunt trip connected to ever voltage, reverse current or over current relays. Such breakers are of rating of to & including 6,000 A a.c

& 12,000 A d.c, voltage ratings are 250 to 600 V a.c & 250 to 750 V d.c. Special breakers available up to 3,000 V for d.c services.

C. Air Blast Circuit Breakers:

The air blast circuit breakers employs compressed air (at a pressure of 20 kg/c.m2) for arc extinction & are finding their best application in systems

operating 132 kV & above (up to 400kV) with breaking capacity up to 7,500 MVA (during short circuit fault)& above, although such breakers have also been

designed to cover the voltage range of 6,600 Volts to 132,000 Volts. These breakers have the advantages of less burning of contacts because of less arc energy, little maintenance , facility of high speed reclosure, no risk of explosion

& fire hazard & suitability for duties requiring frequent operations. The drawbacks of such breakers are additional need of compressor plant for

supplying compressed air, current chopping, sensitivity restriking voltage & air leakage at the pipe line fittings.

27

D. Vacuum Circuit Breakers: The idea behind the vacuum circuit breakers is to eliminate the medium between

the contacts-vacuum. The dielectric strength of vacuum is 1000 times more than that of any medium. In construction it is very simple circuit breaker in

comparison to an air or oil circuit breakers. These breakers are used for reactor switching, transformer switching, capacitor bank switching where the voltages

are high & the current to be interrupted is low.

E. Sulphur Hex-fluoride Circuit Breakers: SF6 gas has unique properties, such as very high dielectric strength, non-reactive to the

other components of circuit breakers, high time constant & fast recombination property after removal of the source energizing the spark, which proves it superior to the other

mediums (such as oil or air) for use in circuit breakers. SF6 circuit breakers have the advantages of very much reduced electrical clearances,

performance independent of ambient conditions, noise less operation, reduce moisture problem, minimum current chopping, small arcing time, no reduction in dielectric strength of SF6, low maintenance, reduced installation time & increased safety. Such as

circuit breakers are used for rated voltages in the ranges of 3.6 to 760 kV. For the later operation a relay wt. is used with a C.B. generally bulk oil C.B. are used

for voltage up to 66 KV while for high voltage low oil & SF6 C.B. are used. For still higher voltage, air blast vacuum or SF6 cut breaker are used. The use of SF6 circuit

breaker is mainly in the substations which are having high input kv input, say above 132kv and more. The gas is put inside the circuit breaker by force ie under high

pressure. When if the gas gets decreases there is a motor connected to the circuit breaker. The motor starts operating if the gas went lower than 20.8 bar. There is a

meter connected to the breaker so that it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gas to reduce the torque produce in it due to any fault in

the line. The circuit breaker has a direct link with the instruments in the station, when any fault occur alarm bell rings.

Protective relay

A protective relay is a device that detects the fault and initiates the operation of the C.B. is to isolate the defective element from the rest of the system”. The relay detects

the abnormal condition in the electrical circuit by constantly measuring the electrical quantities, which are different under normal and fault condition. The electrical

quantities which may change under fault condition are voltage, current, frequency and phase angle. Having detect the fault, the relay operate to close the trip circuit of C.B.

There are two principle reason for this; Firstly,if the fault is not cleared quickly, it may cause unnecessary interruption of service to the customer. Secondly, rapid

28

disconnection of faulty apparatus limits the amount of damage to it & a prevents the effects from speeding into the system. A protective relay is a device that detects the

fault & initiates the operation of circuit breaker to isolate the defective element from the rest of the system. Most of the relays operate on the principle of electromagnetic

attraction or electromagnetic induction. The following important types of relays are generally used in electrical distribution & transmission line:

1. Induction Type Over Current Relay

2. Induction Type Over Voltage Relay

3. Distance Relay

4. Differential Relay

5. Earth Fault Relay

1. Induction Type Over Current Relay: This type of relay operates on the principle of electromagnetic induction initiates corrective measures when current in the circuit

exceeds a predetermined value . The actuating source is a current in the circuit supplied to the relay by a current transformer . These relays are used on ac circuits only and can

operate for fault flow in either direction. Under normal condition the resulting torque is greater than the driving torque produced

by the relay coil current. Hence the Aluminum disc remains stationary, by during fault current in the protective circuit exceeds the preset value. The driving torque becomes

greater than the starting torque & the disc starts to rotate, hence moving contact bridges are fixed contact when the disc rotates to a preset value. Trip circuit operates the circuit

breaker, which isolates the faulty section.

2. Induction Type Over Voltage Relay: This type of relay operates on the principle of electromagnetic induction & initiates corrective measures when current in the circuit exceeds a predetermined value. Under normal condition the aluminum disc remains

stationary. However if the voltage increases at any cost the disc starts to rotate, hence moving contact bridges to the fixed contact when the disc rotates through a preset

angle. Trip circuit operates the circuit breaker, which isolates the faulty section.

3. Distance Relay: Under normal operating condition, the pull is due to the voltage element. Therefore the relay contacts remains open. However when a fault occurs in

the protected zone the applied voltage to the relay decreases where the current increases. The ratio of voltage to current faults is below the predetermined value.

Therefore, the pull of the current element will exceed that due to voltage element & this causes the beam to tilt in direction to close the trip circuit.

29

4. Differential Relay: It compensates the phase difference between the power

transformer’s primary & secondary. The C.T.s on the two sides are connected by pilot wires at both ends are same & no current flows through the relays. If a ground or phase-to-phase fault occurs, the currents in the C.T.s no longer will be the same & the differential current flowing through the relay circuit will clear the breaker on both sides of transformers. The protected zone is limited to the C.T.s on the low voltage side & C.T.s on the high voltage side of the transformer. This scheme also provides protection for short circuits between turns of the same phase winding. During a short circuit, the turn ratio of power transformer is altered & cause

unbalance in the system which cause the relay to operate. However, such sorts are better taken care by Buchholz relay.

5. Earth Fault Relay: This scheme provides no protection against phase to phase faults unless & until they develop into earth faults. A relay is connected across

transformer secondary. The protections against earth faults are limited to the region between the neutral & line current transformer.

Under normal operating condition, no differential current flows through the relay.

When earth fault occurs in the protected zone, the differential current flows through the operating coil of the relay. The relay then closes its contacts to disconnect the

equipment from the system.

Protection Against Lightening:

Transients or Surges on the power system may originate from switching or other causes, but the most important & dangerous surges are those which caused by

lightning. The lightning surges may cause serious damage to the expensive equipments or strokes on transmission lines that reach the equipments travelling as a wave. Thus it

is necessary to provide a protection against lightning surges. They are:- 1. Earth Screen.

2. Overhead Ground Wire.

3. Lightning Arrestor.

1. Earth Screen: The power stations & the substations are generally have much

expensive equipments. These stations can be protected from direct lightning strikes by providing earthing screens. It consists of a network of Copper conductors mounted all

30

over the electrical equipments in the substation or Power station. The screen is properly connected to earth on at least two points through low impedance. On the

occurrence of direct stroke on the station the screen provides a low resistance path by which lightning surges are connected to the ground. In this way station equipments are

protected against lightning.

2. Overhead Ground Wires: The most effective method of providing protection against direct lightning strokes is by the use of overhead ground wires. The ground

wires are placed over line conductors at such position that practically all lightning strokes are intercepted by them. The ground wire is ground at each tower or pole

through as low resistance as possible. When the direct lightning strokes occur on the transmission line will be taken you by the ground wire. The heavy current flows to the

ground through the ground wire, so it protects the line from harmful effects of lightning.

3.Lightening Arrestors: Firstly, we can see lightning arrestors. These lightning

arrestors can resist or ground the lightning, if falls on the incoming feeders. The lightning arrestors can work in an angle of 30 degrees around them. They are mostly

used for protection of the instruments used in the substation. As the cost of the instruments in the substation are very high to protect them from high voltage lightning

these arrestors are used. It is a device used in Electrical Power systems to protect the insulation o the system

from the damaging effect of lightning. Metal Oxide arrestors (MOVs) have been used for power system protection the mid 70s.The typical lightning arrestor is also known

surge arrestor has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrestor, the current

from the surge is diverted around the protected insulation in most cases to earth. Lightning arrestors with earth switch are used after the current transformers to protect it from lightning i.e. from high voltage entering into it. This lightning arrestor has an

earth switch that can directly earth the lightning. The arrestor works at 30o to 45o angle of the lightning making a cone. The earth switch can be operated manually, by pulling

the switch towards the ground. This also helps in breaking the line entering the station. By doing so maintenance repair of any instrument could be performed.

Types of lightning arrestors:- There are several types of lightning arrestors are in use,

differs only in their constructional detail but they are electrically identical & operate on the same principle.

They are-

31

a. Rod gap arrestor

b. Horn gap arrestor

c. Valve type arrestor